By Bruce Rottink, Volunteer Nature Guide & Retired Research Forester
All photos by the author unless noted.
The forest at Tryon Creek State Natural Area (TCSNA) is literally rooted in the soil. What happens below the surface of the ground is vitally important to the forest, but something which is generally hidden from us. Rarely, Mother Nature provides us with a glimpse into her underground world, and it can be very enlightening.
The Root System Revealed
In late September of 2013, a little more than 5 years ago, a tree growing by the side of the road leading up to the “Horse Lot” at TCSNA blew over during a rainy, stormy period. Seizing the opportunity, I started studying this event, with the idea of creating a series of Natural Notes possibly stretching over many years.
When the tree blew over, the root system was revealed. Photos taken within 2 or 3 days of the blowdown are shown below.
After the tree fell over, for at least a couple of days afterwards, there was a pool of water covering the clay layer exposed when the tree had fallen over. It turns out that the soil under this tree, just like most of the soil at TCSNA, has only about a 2 to 3 foot layer of soil suitable for growing plants, underlain by a thick layer of clay. This clay is very resistant to water flow and root growth. For safety reasons, the Park Staff filled in this hole shortly after the tree fell over. The photo below gives an indication of the size of the root wad. The red arrows show moderate- to large-sized roots growing horizontally, not downwards, because they hit the clay layer.
— Photo by Anonymous Park Visitor
I measured the thickness of the root wad within a week of when the tree fell over. I did this by pounding a thick metal rod through the root wad, and measuring how much of the rod stuck out of the soil. A picture of this method is shown below. I pushed the rod through the root at 4 different locations, both 2 and 3 feet on either side of the trunk of the downed tree. On average, the thickness of the root wad was 24-1/2 inches.
As you can see, there are no roots growing straight down out of the root mass. The clay layer beneath this tree was not hospitable.
The Aging Root Wad
Five years after the tree blew down, I returned to the site, and measured the thickness of the root wad again, in the same way I had measured it when the tree first fell down. In the 5 years since it fell over, the soil on the root wad was 4.9 inches thinner than it was when it first fell down. Based on other trees that have fallen over in the forest, like this one along the Maple Ridge Trail shown below, I anticipate that sooner or later, all the soil will be washed off the skeletal root system.
An Underground Dam
These root systems all raise questions about the depth of the “plant friendly” soil, which in much of the park, seems to be pretty shallow. In many cases of fallen trees, the soil which is exposed is substantially clay. Clay of course is resistant to water flow. Just how resistant? I collected a sample of clay from the root system of a tree that had recently fallen at TCSNA.
At home, I drilled holes in the bottom of a plastic cup, as shown below. Water flowed easily through the holes as you can see in the picture below.
For my test, I put about 1/3 of an inch of clay into the pot, and gently pressed it down into the pot. Then I filled the pot with water. There was some tiny amount of water that flowed through the holes but not much. I let the pot sit with water in it for a couple of days. Then I once again filled the pot with water and let it sit inside of a plastic tray. I sat the pot on two pencil stubs to keep the bottom of the pot up off the tray, so water could easily run out of the holes. This is illustrated below. I left out the plastic tray in order that you could see the rest of the set-up more easily.
In the course of 20 hours, not a single drop of water leaked out of the cup. The clay used in this demonstration is clay that is found underground throughout much of TCSNA.
The nearly impenetrable layer of clay found at TCSNA means that the forest we love is dependent on approximately the top two feet of soil. To put this in perspective, when leading hikes for students at the Park, I will oftentimes ask them this question: What would they think if I went to Washington Square and dumped 2 feet of dirt on top of the asphalt parking lot, and declared that I was going to start growing a forest there? Almost always the kids will say something like “You are crazy!” But in fact, that is essentially the situation we have here at the Park.
The photo below shows me with a cardboard box the same height (24.5”) as the depth of soil supporting the trees at TCSNA. This is the depth of soil I would pour onto the parking lot in order to create a forest at Washington Square Mall like that at TCSNA.
The thin layer of soil supporting the forest at TCSNA is one reason that the trees need to shelter each other if they are going to resist being blown down by the wind. They really do constitute a “Forest Community.”
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
Following my recent Naturalist Note on water I was persuaded to write a follow-up note, and this is it!
Water is a key to life, for both plants and animals. When you think of water at Tryon Creek State Natural Area (TCSNA), you naturally think of the creek. But that’s a long way from the whole story. In a Naturalist Note1 published earlier this year, I included the following image of how much water was in the top 12” of soil at TCSNA in late April based on 22 soil samples.
Just as a reminder, I need to stress that this is only in the top 12” of soil. In much of the park, water available to the plants might be located as deep as 2.5 feet beneath the surface. At approximately 2.5 feet beneath the surface is a layer of clay which is relatively impermeable to plant roots (more on this in my next Naturalist Note.)
During the course of this summer, there was not much rainfall, as illustrated in the graph below, which is based upon rainfall at two local stations, one at the Lake Oswego City Hall, and the other at the Westlake Fire Station.
Rainfall April 23 to Aug 23, 2018
As documented in a recent Naturalist Note, light rainfalls such as the majority of those illustrated in this graph, may primarily end up in the crowns of trees and shrubs, and never even make it to the forest floor.
On August 22 and 23, 2018 I collected soil samples from approximately the same locations where I sampled the soil in April. I again took samples from only the top 12 inches. The results were stunningly different. Below is a diagram using the same number of pools which I described in the spring. This time, however, 42 of the pools are dry, and only 26 are full of water.
The number of Olympic-sized swimming pools that could be filled with water from the top 12 inches of soil found in late August 2018 at Tryon Creek State Natural Area.
Where did it go?
There are probably three main fates for the missing water in the top 12” of soil. First, any water at or close to the surface could simply have evaporated. I have no idea how much this would amount to, but in the top 1 to 2 inches of soil, I’m guessing this could be an important factor.
Subsurface Water Flow
Secondly, the water particularly in sloped areas, could have flowed down hill underground. Lateral underground movement of water is quite common. To illustrate this I poured several gallons of water onto the soil surface at a flat spot on the side of the Old Main Trail not too far from the Nature Center. After saturating the soil in this tiny area, I used a soil corer to create two 6” deep holes about an inch in diameter. I waited for several minutes until there was no freestanding water in either hole. Then I carefully poured water into the right hand hole, as seen in the picture below. Not surprisingly, the water flowed laterally underground, and appeared the other hole.
This subsurface water flow might be particularly important on the steep hillsides near the creek. To illustrate this effect, I found information online about two different watersheds. The first is the Tryon Creek watershed, which is the watershed in which the TCSNA is located. An analysis of this watershed by the City of Portland has shown that about 25% of it is made up of impermeable surfaces, like rooftops, sidewalks, driveways, streets, tennis courts and even swimming pools. An aerial photo of one small part of the watershed illustrates the extent of these impermeable surfaces.
Water falling on these impervious surfaces rapidly makes its way into Tryon Creek, thanks in part to storm sewer drains that are common in the city.
In contrast, the Fir Creek watershed, located in the vicinity of the Bull Run Reservoir in the foothills of the Cascades east of Portland is almost completely forested, with the only impermeable surfaces being a few roads in the area. These two watersheds are roughly similar in size.
The differences in the surfaces of these two watersheds creates an enormous difference in the water flow in the major creeks of the watershed. These differences are illustrated using data from the same “rain event” in both watersheds.
In this graph you can see that immediately after each large rainfall event that there is a sharp peak in the water flow in Tryon Creek. This sharp peak is followed by a slow decline in the water flow of the creek. It seems reasonable that the brief sharp peak in the creek depth is water running off the impermeable surfaces found in the watershed, and being quickly dumped into the creek by the storm sewer systems. The slower decline following the sharp peak, is, I assume, water actually slowly flowing through the soil and into the creek.
In contrast to the water flow in Tryon Creek, in the Fir Creek system, there is a slow but significant increase in the stream flow following the rains. I suspect this is because the water in this watershed all has to gradually seep through the soil, and slowly make its way down to the creek.
Water Usage by Plants
Thirdly, the water could have been extracted from the soil by the roots of plants, and subsequently evaporated from plant leaves. This would be one way in which soil water at some depths could be lost to the atmosphere. In studies2 of water usage by Douglas-fir (Pseudotsuga menziesii), for example, it was reported that a 60-foot tall tree with an 8 inch diameter used 5 gallons of water per day. A 91 foot tall Douglas-fir with a 14 inch diameter used 16 gallons per day. Our forest has numerous trees this big and bigger, so their water usage in summer for the forest as a whole could be more than we might first imagine.
Living as we do in an area with relatively dry summers and wet winters we could see dramatic changes from climate change. If we have a winter with subnormal amounts of rain, and warmer than average summers, there could be a large die off of moisture-loving plants which have lived in this area for some time. They will of course, be replaced with other plants, but the transition could be difficult.
2Wullschletter, Stan D., F. C. Meinzer and R. A. Vertessy. 1998. A review of whole-plant water use studies in trees. Tree Physiology.
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
All photos by Bruce Rottink.
Slugs are a common sight at Tryon Creek State Natural Area (TCSNA). Just by chance, I’ve personally seen slugs at TCSNA every month except January and February. In November 2014, Tryon Creek Nature Guide Sharon Hawley wrote a Naturalist Note on slugs. You can read it here. This current note does not repeat the wonderful information that Sharon provided.
Banana slugs (Ariolimax columbianus) are a common species found at TCSNA. They are one of the slug species that are native to the Pacific Northwest. They were given their scientific name in 1851. These are not the slugs that typically cause problems in most people’s gardens.
Banana slugs are pretty low down in the food chain, and according to documentation on the web, are sometimes eaten by raccoons, garter snakes, ducks and even salamanders.
What is Slug Food?
Slugs eat a variety of things, including both living and dead vegetation. You can see slugs crawling on many types of plants. The picture below shows a slug on a stinging nettle (Urtica dioica) plant at TCSNA. I was a little surprised to see it there.
Below is a picture of a slug banquet I discovered occurring on the West Horse Loop Trail (and remembering the word “horse” is important here.)
Frequently I lead student nature hikes at the park, and oftentimes I’ve heard of kids attending an outdoor camp where they are encouraged to lick slugs. Just thinking of this picture ensures that I personally will never lick a slug. But enough of that!
The picture below shows a slug which climbed about 3 feet up the side of a tree near the Old Main Trail in order to get a taste of this mushroom. According to reports in the literature, mushrooms are one of their favorite foods.
The slug (of unknown species) in the photo below, is deeply exploring a trillium (Trillium ovatum) blossom. It appears that this slug may have chewed off substantial chunks of the trillium’s petals. Now it appears to be going out whole-hog for the core of the flower.
The banana slugs at TCSNA have lots of things to eat and explore.
Slugs are slow moving creatures, and I can’t imagine what predator they could successfully run away from. So slugs have chosen to employ camouflage. While the vast majority of the banana slugs at TCSNA have black spots, scientists who have studied the species have reported that a few, like the one below, do not.
Camouflage, with or without spots, means that you have to look like something else, or at least blend in to the environment. Below are several pictures of things I’ve seen on TCSNA trails that I’ve momentarily confused with a slug.
I can’t even begin to imagine how many banana slugs live at TCSNA. However, we must admit they are very successful, and make a contribution to the Park by helping turn plant material, both dead and alive, into soil.